1. Field of the Invention
This invention relates to a method for casting molten metal articles within a casting mold utilizing a vaporizable chill member disposed within the mold for controlling solidification of the metal so as to reduce the formation of solidification shrinkage porosity defects within the resultant cast article.
2. Description of the Related Prior Art
As molten metal is cast into a mold and begins to cool, the outer portion of the casting adjacent the mold wall solidifies and forms a skin or shell of solidified metal essentially capsulating the remaining molten metal therewithin and fixing the volume of the casting. As the remaining molten metal further cools and solidifies, it shrinks and, if left uncontrolled, creates shrinkage holes or voids internally within the resultant casting.
To compensate for this shrinkage, many known casting processes are known to employ some source of additional molten metal (i.e., a riser) which is fed into the various sections of the casting as they solidify. This additional metal fills the shrinkage voids as they are formed and produces a sound, porosity-free cast article.
For the riser to be effective, it is important to control the solidification rates of the various sections of the casting or else the thin sections will naturally cool and solidify sooner than the thick sections and freeze off the central, still-molten regions of the thicker sections from access to the additional riser metal.
The broad concept of using a chill to accelerate solidification of select regions of a casting is known to the art as exemplified by the U.S. Pat. No. 2,294,170 to Francis et al, granted Aug. 25, 1942. This patent teaches delivering solid metal chill bodies into the mold after the molten metal has been introduced therein in order to increase the solidification rate of the center of the casting. These chill bodies either melt and combine with the surrounding molten metal or remain solid and become bonded to the casting metal as it solidifies. Both may be undesirable, as dissolving the chill bodies may contaminate the melt and allowing the chills to remain in tact could affect the integrity of the casting.
The U.S. Pat. Nos. 2,750,641 to Raible, granted Jun. 19, 1956; and 4,706,732 to Ruhlandt et at. granted Nov. 17, 1987, disclose using a metal chill body for controlling the solidification rate of various sections of a casting. These chill bodies, however, either remain in tact with the casting or must be removed after casting.
Thus, there is a need in the industry for a casting process utilizing a chill which effectively accelerates solidification of select regions of a cast article as it solidifies to eliminate solidification shrinkage but does not contaminate the casting metal, affect the integrity of the casting, nor add to the expense of manufacturing the casting by requiring removal of the chill bodies after casting.